Three distinct optical-switching states in phase-change materials containing impurities:From physical origin to material design

Ge2 Sb2 Te5 is the most widely utilized chalcogenide phase-change material for non-volatile photonic applications,which undergoes amorphous-cubic and cubic-hexagonal phase transition under external excitations.However,the cubic-hexagonal optical contrast is negligible,only the amorphous-cubic phase transition of Ge_(2)Sb_(2)Te_(5) is available.This limits the optical switching states of traditional active displays and absorbers to two.We find that increasing structural disorder difference of cubic-hexagonal can increase optical contrast close to the level of amorphous-cubic.Therefore,an amorphous-cubichexagonal phase transition with high optical contrast is realized.Using this phase transition,we have developed display and absorber with three distinct switching states,improving the switching performance by 50%.Through the combination of first-principle calculations and experiments,we reveal that the key to increasing structural disorder difference of amorphous,cubic and hexagonal phases is to introduce small interstitial impurities(like N)in Ge2 Sb2 Te5,rather than large substitutional impurities(like Ag)previously thought.This is explained by the formation energy and lattice distortion.Based on the impurity atomic radius,interstitial site radius and formation energy,C and B are also potential suitable impurities.In addition,introducing interstitial impurities into phase-change materials with van der Waals gaps in stable phase such as GeSb_(4) Te_(7),GeSb_(2) Te_(4),Ge_(3)Sb_(2) Te_(6),Sb_(2)Te_(3) will produce high optical contrast amorphous-metastable-stable phase transition.This research not only reveals the important role of interstitial impurities in increasing the optical contrast between metastable-stable phases,but also proposes varieties of candidate matrices and impurities.This provides new phase-change materials and design methods for non-volatile optical devices with multi-switching states.

New design for highly durable infrared-reflective coatings

The fundamental challenge in designing durable infrared-reflective coatings is achieving the ideal combination of both high reflectivity and durability.Satisfying these competing demands is traditionally achieved by deposition of durable layers on highly reflective metals.We overturn the traditional logic of‘first reflectivity and then durability’and propose an alternative of‘first durability and then reflectivity’:First,a transition-metal compound is selected as a durable base;then its reflectivity is improved by incorporating silver/gold to form an alloy or by overcoating a multilayer stack.Two validation experiments prove that the new strategy works extremely well:the coatings thus obtained have infrared reflectivities close to that of aluminum,and their hardness and acid and salt corrosion resistances are 27–50,400–1500 and 7500–25000 times that of aluminum.The traditional mirror coating(e.g.,Al/SiO2 films)is more suitable for moderate environments,while our mirror coating that was obtained by the new strategy(e.g.,an Ag-doped hafnium nitride film)is more suitable for harsh environments,such as ones with dust,windblown sand,moisture,acid rain or salt fog.This work opens up new opportunities for highly durable infrared-reflective coatings and rejuvenates the study of transition metal compounds in a completely new area of optics.